1. Field of the Invention
Systems and methods for improving the throughput and the utilization of monorail vehicles bearing a work-in-progress are disclosed; and, more particularly, systems and methods for logically-transporting vehicles bearing a work-in-progress on an monorail factory system using asynchronous conveyor line segments.
2. Description of the Related Art
In semiconductor manufacturing environments, work-in-progress lots must sequence through one or more process steps that are performed by a tool specific to that process, i.e., a process tool. The sequencing of each lot to and through the required process tool is often recursive, which is to say, that a lot may return to the same process tool plural times.
In the last ten years, the semiconductor manufacturing industry has undergone significant technology change. For example, to increase productivity, the diameter of the silicon wafer was increased from 200 mm to 300 mm. As a result, due to the increased size and weight of the wafer lot carrier, a plastic enclosure called a FOUP or front opening unified pod was designed to hold wafers securely and safely and, further, to provide access to the stored wafers using robotic handling systems.
Manufacturers of semiconductors also agreed to standardize logistics hardware, choosing a technology for transporting wafer lots between process steps that is based on ceiling-mounted monorail vehicles. The decision to standardize semiconductor manufacture using monorail vehicles and a monorail transportation system, however, includes several inherent shortcomings.
For example, as one would imagine, a single, uniform technology cannot satisfy all of the requirements for modern transport needs efficiently. Consequently, by committing the industry and semiconductor manufacturers to a single, uniform transportation technology, suppliers of monorail transports have been forced to develop costly solutions within that technology, in an attempt to comply with industry demands for higher performance.
For example, modification costs incurred can be significant because factory assets, e.g., clean facilities and process equipment, are very expensive. Moreover, the adopted vehicle technology is limited in its capacity to deliver work pieces. Indeed, inherent in the design of a monorail transportation system is a limitation on vehicle utilization. For example, referring to FIG. 1, currently, each vehicle can only carry a single FOUP. Moreover, within the domain of the monorail transportation system, each vehicle, as it circumnavigates the circuit of the vehicle loop 1, can only be scheduled for sequential pick up 2 and delivery or drop off 4 of the FOUP. Between the pick up point 2 and the drop off point 4, the vehicle experiences a useful run 3. However, after delivery 4 and before the next pick up 2, the vehicle is unused.
As a result, in this two-step, linear or quasi-linear process, vehicles will sometimes run empty 6, e.g., after drop off 4 and before the next pick up 2, and/or vehicles will sometimes be idle 8, corresponding to the time the vehicle waits before its next pick up assignment after drop off 4. Whenever a vehicle is idle or empty, the over-all utilization rate of the system is reduced, which affects the system's capacity to deliver. The adverse effect of such limitations cascades down to the utilization of the semiconductor process equipment, e.g., the process tools, as well as to the time, or, more specifically, the cycle time, it takes for the semiconductor product to transit through all factory processes to finish.
To address these shortcomings using the standardized monorail technology requires finding a proper balance between providing more vehicles and increasing the vehicular speed. These solutions, however, are expensive and also have practical limits. Chief of which is that vehicle numbers or quantity and vehicle speed are inversely related.
For example, at some point, the law of diminishing returns limits the number of vehicles that can be added to the transportation system. The number of vehicles that can be added depends on the available length of the transportation system, the speed of transit of each vehicle, and the requirement for collision-free, stop-and-go operation. When a greater number of vehicles are employed on a common track, the distance between vehicles decreases. As a result, the vehicle speed must be lowered to ensure that the resulting distance between vehicles provides adequate room for deceleration to ensure collision-free, stop-and-go operation. In short, higher speeds require longer stopping distances because of practical deceleration limits. If the necessary stopping distances are longer, then fewer vehicles can operate on the transportation system.
Therefore, it would be desirable to maximize the efficiency of discrete vehicles operating on a standardized monorail transportation system by integrating peripheral conveyor line segments into the standardized monorail transportation system. Advantageously, by integrating peripheral conveyor line segments into the existing transportation technology, the performance of the existing system can be improved and vehicle utilization and throughput can be increased. More specifically, the conveyor line segments provide dynamic, asynchronous vehicle transfer and buffering points that can be used to maximize vehicle utilization and throughput.